Casting molds



Patented May 16, 1950 CASTING MOLDS Raymond E. Birch, Pittsburgh, Pa.,assignor to 5 Harbison-Walker Refractories Company, Pittsburgh, Pa., acorporation of Pennsylvania No Drawing. Application February 21, 1947,Serial No. 730,189

1 This invention relates to molds and mold parts of the sand, ornon-permanent, type for the casting of metals and alloys, moreparticularly those cast at temperatures of at least about 1400 C. Unlessthe contrary appears from the context, the word sand is used herein torefer not only to silica sand, crushed quartz and the like materialscomposed largely or substantially wholly of silica ($102), but alsobroadly includes other refractory materials that are used in subdividedform for the making ofmetal casting molds and mold parts. A variety ofsuch other materials are known, examples being magnesite, zircon andolivine.

A major object of the invention is to provide refractory molds and moldparts, such as cores,

for the making of metallic castings, that are easily prepared inaccordance with standard practices, and which are characterized byundergoing dusting, or mechanical disruption, of the refractory sandadjacent the formed casting whereby stripping of the casting isfacilitated and adherence of the refractory to the surface of thecasting is minimized or eliminated.

A further object is to provide refractory compositions for making moldsand mold parts, or for the surfacing of metal-contacting areas thereof,which are characterized by possessing the properties requisite for suchuses and additionally of undergoing dusting in the regions adjacent theformed casting during and after cooling of the cast metal.

Still another object is to provide an improved method of casting metalsand alloys, particularly those melting or castable above about 1400" C.,which results in dusting of the mold material in contact with the castmetal during or after cooling whereby to insure easy stripping of thecasting with minimized adherence of the mold material thereto.

Yet another object is to provide compositions, molds and a method inaccordance with the foregoing objects that are applicable not only tosand type molds but also to the casting of metals and alloys in otherways including, by way of example, dental and the like investmentcastings.

Other objects will be recognized from the following specification.

Calcium orthosilicate, or dicalcium silicate (2CaO.SiOz) is ahighlyrefractory material capable of existing in three allotropicmodifications, i. e., an alpha crystal form that is stable above about1420 0., a beta form that is stablein the range from about 1420 to 6750., and a gamma modification that is stable below about 4 Claims. (Cl.22-188) 675 C. As has been known for many years, the inversion from thebeta to the gamma allotroplc modification involves an increase in volumethat apparently takes place instantaneously and which results inmechanical disintegration of the crystal grains, a phenomenon known tothose familiar with it as dusting.

In the refractory art the presence of calcium orthosilicate inrefractory materials has been considered undesirable because of thetendency to disruption of refractory shapes when they are cooled to orbelow the beta-gamma inversion temperature, and the presence of thismineral has been tolerated in refractories only when stabilized againstthat inversion by certain oxides such, for example, as C1203, P205,B203, V205. and M11203.

Despite the prior beliefs that the dusting of calcium orthosilicate isundesirable and objectionable in refractories, I have found, and it isupon this that the invention is in large part predicated, that thisphenomenon is advantageously utilizable in the casting of metals innon-permanent, or sand, molds. More particularly, I have found that thestated objects of the invention are attained by having calciumorthosilicate in an unstabilized condition present in at least themetal-contacting surfaces of refractory molds whereby after the castmetal has cooled to or below the temperature at which the beta, forminverts to the gamma form, the refractory material adjacent the castingwill be disintegrated, or dusted. The significance of thisself-powdering of the refractory material adjacent the casting is thatit renders stripping of the castings from the molds easier, and thecastings are freer y from adherent refractory than is the case withmolds of the same class used heretofore. Accordingly, the inventionresults not only in more rapid and easier stripping of castings from themolds than heretofore, but also, the previously necessary laborious andtime-consumingoperation of cleaning the castings to remove adherentrefractory is reduced, or even eliminated.

The invention may be practiced in various ways. Thus, themetal-contacting surfaces, or

mold cavities, of ordinary silica sand or other non-permanent types ofmolds made from refractory materials may be surfaced with materialsupplying unstabilized calcium orthosilicate in accordance with theinvention. Or, the entire mold or mold parts, such as cores, may be'madefrom refractory substances adapted to provide unstabilized calciumorthosilicate under the heat of the cast metal. In all such embodimentsthe mold or the facing material may consist wholly of materials reactiveto provide calcium orthosilicate, or it may be made up of refractorymaterials adapted to provide certain predetermined proportions of theunstabilized silicate.

Similarly, the unstabilized calcium orthosilicate may be provided invarious ways, as will appear hereinafter.

In the preferred practice of the invention unstabilized calciumorthosilicate is formed by reaction in situ in the mold substance underthe heat supplied by the cast metal. Thus, metal casting molds and partsthereof may be made from refractory material containing lime and silicain forms and in amounts such that unstabilized calcium orthosilicatewill be formed when metal is cast in the mold and will undergo thebeta-to-gamma inversion upon cooling the casting in the mold.

I now believe that with materials productive of calcium orthosilicate inthe forms in which they would most commonly be used in molds react at arate satisfactorily productive of effective amounts of the orthosilicateat temperatures over about l300 or 1400" C., and that consequently, theinvention is particularly applicable to those metals and metal alloysthat are normally cast at such temperatures, such as ferrous alloys andthe precious and the high melting metals and their alloys, or othermetals or alloys that may be cast satisfactorily at such temperatures.Nevertheless, it is entirely possible for this orthosilicateto be formedat lower temperatures, and particularly in the case of materials instates conducive to enhanced activity such, for example, as slaked limeand silica in an active form, e. g., precipitated silica or silica gel.Accordingly, the invention is not restricted to cases where the metal iscast at or above l300 or 1400 C. but is rather applicable whereversufficient calcium orthosilicate can be supplied to effect the desireddusting.

Calcium orthosilicate is theoretrically composed of 65 per cent of CaOand 35 per cent of S: and for the purposes of the invention therefractory material consists of or contains those constituentssubstantially in those proportions. However, as will appear, theproportions may be varied somewhat from that theoretical composition. Itis important, and critical, however, that however supplied the limitsindicated by the data given hereinafter be not extended substantiallyfor if they are, other calcium silicates may be formed in amounts thatinterfere with or prevent the dusting action that is essential to theinvention. This refers, of course, to the actual material productive ofdusting, i. e., the calcium orthosilicate. For example, this may be dueto particles of lime or of silica converted throughout to theorthosilicate. Equally, however, sufiicient of the orthosilicate may beformed on the surfaces of granules, e. g., of quartz, to cause thedesired dusting on cooling; in such a case it is the composition of theorthosilicate coating that is important, not the overall composition ofthe particle which may be very different.

In general, as little as 10 per cent by weight of unstabilized calciumorthosilicate suflices to effect adequate dusting of various refractorysands, as that word is used herein, but larger amounts are generallypreferable, and the refractory may even be converted wholly to theorthosilicate, particularly when applied merely as a mold facing.

The most obvious example of a suitable composition for practicing theinvention is to mix aaoaooo lime (C'aO) and silica (810:) in proportionsproductive of calcium orthosilicate. For most purposes, however, it ispreferable to adjust the composition of refractory materials adapted tothe production of sand-type molds by the addition of one or morematerials that provide unstabilized calcium orthosilicate in therefractory and which exert no specific influence on the molds or thecastings, as by blending refractory materials containing lime andsilica, naturally or added, so that they are present in proportions andamounts productive of enough calcium orthosilicate to cause the desireddusting.

The latter procedure can be illustrated by reference to the blending oftwo dead burned magnesites, designated A and B, whose analyses are givenin the following table. In the blending of those magnesites to yieldunstabilized calcium orthosilicate it is advisable. as indicated, to aimat a mixture containing lime and silica in the ratio of 65:35 by weight.By calculation this requires 30 per cent of magnesite A and 70 per centof magnesite B, and the resultant blend will have the analysis indicatedin the table as composition C. This gives a total of lime-plus-silica of18.9 per cent, which is the maximum amount of dicalclum silicate thatcan form in the blend.

A Stan d 0 Analysis High Lime at Magneslte washingtgl Percent PercentPercent BIO, 4. 90 7. 35 6. 62 Ratio of Coo/S101 (approx). /15 43/5765/35 ceramic art to refer to the total content of sesquioxides, in thiscase alumina and iron oxide reported as FezOa. The term is used in thisway in, for example, Patents Nos. 2,278,454 and 2,292,644, as well as atpages 50 and 51 of Ceramic Tests and Calculations by A. I. Andrews(1928).

Experience has shown that with dead burned magnesites the practicalrange in CaO SiOz ratio is from 68:32 to 60:40, but little more. As willappear, the range may be somewhat wider with other refractories.

In the practice of the invention molds and mold parts, such as cores,are made in the manner customary in the trade, there being nothing inthe nature of the compositions used that requires any change fromexisting sand mold practices. Thus, no change is necessary ordinarily inthe grain size or range of grain sizes of the refractories, or sands,and the standard bonding agents may be used including such well knownbonds as bentonite, clay, eyrite, cement, lignin extracts, celluloselye, molasses, oils, resins, etc.

II is cast in them in the usual manner, and the castings are allowed tocool at least to the temperature at which the dusting occurs, afterwhich they may be stripped readily from the molds and, as indicatedabove, there will usually be no substantial adherence of the refractoryto the castings such as is encountered in the use of ordinary B eachinstance they weremoided into cylinders after adding per cent'of' waterand 2 per cent of lignin liquor to give a temporary bond:

Mix mm Mix #2 Mlx'#3 Magnesite' .percent 80 7 6 70 Cement. do 2) 25 30Calc.Ca0/Si0z(npprox.) 65/35 67/33 69/31 The compositions used in thepractice of the invention are capable of re-use because normally theportion of the mold that dusts will represent only a thin reacted layernext to the metal. If judged necessary to remove the dusted material,this can be done readily by allowing the powder to fall away beforecrushing the mold for re-use, or by screening out the dust. However, thedusted powder will do no harm except that build-up in successive batchesarising from re-use of the molds will in time reduce the permeability ofthe mold to an extent that may be objectionable. Naturally, it is goodpractice when re-using these refractory materials to mix some new batchwith the old before re-using it.

Although, as indicated above, the entire mold may be made fromcompositions in accordance with the invention, it is entirely feasibleto use such compositions only for facing the mold cavity or othermetal-contacting surfaces, such as the surfaces of cores, which will bein direct contact with the casting. Generally speaking, such facings maybe about A to inch thick. This facing layer may extend over the entiremold cavity or only where the greatest trouble due to adherence of sandto castings is anticipated. In this way cheaper sands or otherrefractory materials may then be used for back-up, and in general forthose parts of the mold that do not affect the surface of the casting atvital points. The use of compositions yielding calcium orthosilicate,whether to constitute the entire mold or only a facing as justdescribed, does not interfere in any way with the well-known practicesof using thin wash or brush coatings of graphite, calcined clay, silicaflour, olivine dust, or other materials commonly applied to finishedmold surfaces.

As a. further example of compositions in accordance with the invention,reference may be made to the blending of Portland cement and dead burnedmagnesite, the former of which typically has a CaO:SiO2 ratio higherthan that of dicalcium silicate, and the latter of which has a lowerratio than the desired resultant. In one series of tests there were usedcement and dead burned magnesite of the following composition:

Using these two materials ground to pass 65 The molded cylinders wereheated to 1400 C. to cause the formation of calcium orthosilicate, aswould occur in a, casting operation. The test pieces were then cooled,and upon inspection it was found that mix #1 was cracked, and that itcompletely powdered after 24 hours. Mix #2 powdered completely withoutdelay, while mix #3 did not show dusting for several days. It followsthat in this instance the ratio 67Ca0 I 338102 was better, from thestandpoint of prompt dusting, than the theoretical :35 ratio, and thatthe practical dusting range does not extend to a ratio of 69:31. Thesetests show also that the dusting inversion does not invariably occur atthe normal inversion temperature of 675 C., but that a metastable statemay exist which will delay inversion and dusting for some hours, in viewof which castings made in such molds may clean more readily if there issome delay in stripping them from the molds. The reason why dusting wasmore effective with these mixes at a CaOzSiO2 ratio of 67:33 than at the65:35 ratio is probably to be found in the influence of the minorconstituents of the mix, or it may be due to the magnesia or alumina.

In some instances there would be no objectio to a strong set such as maybe obtained by Portland cement, and in those cases it may beadvantageous to use enough water not only to cause full setting of themix, but enough actually to make molds by casting the mix around thepattern. On the other hand, in other instances the hydraulic action ofthe cement may give a stronger set than desired. This, however, is afamiliar problem in the foundry that is taken care of variously, as bythe known use of a limited amount of water to prevent attaimnent of fullset, or by the use of oils instead of water, which is particularlyfeasible when the compositions are used only for facing the mold.

Although the invention has been described in detail with reference tothe provision of the desired unstabilized calcium orthosilicate byreaction under the heat of the castings, and although it is preferred topractice the invention in that manner, equivalent results within the.

scope of my invention may be achieved simply by providing in the mold orthe mold facing beta calcium orthosilicate held temporarily in anunstabilized metastable form whereby the heat supplied by the cast metalwill cause the inversion to the gamma form to occur, with dusting, uponcooling the casting and mold assembly. Thus,

dead burned magnesites can be produced withabout 20 to 30 per cent ofunstabilized dicalcium silicate held in the beta form almost permanentlyat normal room temperatures if the product is quickly cooled, orchilled, upon emergence from the rotarykiln in which it is produced. Thebeta form will apparently continue indefinitely in such a product unlessand until the magnesite grain is again heated and then slowly cooled.Such a mesh, 3 mixes were made up as follows, and in 7| materialcontaining the beta modification held in a metastable condition as theresult of an initial quick cooling can, of course, be used for theproduction of molds and the making of castings in accordance with myinvention because, to repeat, the heat supplied by the molten metal andthe slow cooling of the mold and its contained casting will causeinversion of the dicalcium silicate to the stable gamma form withdusting of that part of the molding composition adjoining the casting.

It is to be understood that any refractory base material, or combinationof such materials may be used for making the 'molds that supply theproperties requisite for such purposes together with the provision ofenough calcium orthosilicate ,to effect dusting. Suitable ingredientsparticularly for supplying the unstabilized calcium orthosilicate aredolomite, limestone, calcium and magnesium silicates, wollastonite,pyroxene, amphibole, silica sand, sandstone, quartzite, serpentine,olivine and slags. Where raw materials exhibiting high shrinkage areused, such as dolomite and limestone, primary calcination is ordinarilydesirable.

A readily dustable mix has been made, for example, by the blending of56.1 per cent of Mg-Fe olivine sand or dunite from North Carolina with43.9 per cent of hydrated lime to give a mix with a CaO SiOz ratio of65:35. Similarly good results were obtained using dolomitic lime inplace of high calcium lime. Work with mixtures of dolomite and olivinegave the free dusting desired with all CaO S102 ranges from 58:42 to69:31. The olivine used analyzed 39.6 per cent SiOz, 47.6 per cent MgO,8.6 per cent FeO, andonly a trace of CaO. Mixtures of potters flint andcrude dolomite give dusting in' the same range.

As an example of a three component mix in accordance with the invention,there may be used any combination of serpentine, dead burned magnesite,and a calcareous ingredient such as cement or lime provided theproportions are such as to effect the provision of calciumorthosilicate, suitably approximately 65 per cent CaO and 35 per centSiOz. If the magnesite constitutes about per cent, the mixes will befree from excessive shrinkage.

There may be used also as a mold material a mixture of silica sand, or asilicate, and a prefired calcium ferrite clinker.

Although the invention has been described thus far with particularreferences to sand-type molds, it is to be understood that the making ofcastings, such as dental castings, of the precious or tage of thepresent invention for such purposes is that the presence of CaO appearsto prevent sulfur damage of the castings. The compositions used in thepractice of the invention are applicable also with advantage in thefusion casting of refractories, particularly those of basiccompositions.

According to the provisions of the patent statutesJ have explained theprinciple and mode of practicing my invention, and I have described whatI now consider to represent its best embodiment. However,- I desire tohave it understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically described.

I claim:

1. A refractory mold or mold part for the casting of metal having atleast the metal-contacting surface portion composed of refractorymaterial containing, by chemical analysis, lime and silica substantiallybetween the weight ratios 58Ca0 428102 and 6902.0 BlSiO: and reactivewhen heated by the cast metal to at least about 130 0 C. to provide atleast about 10 per cent of unstabilized calcium orthosilicate based onthe weight of said material, said orthosilicate undergoing dusting atthe surface of the casting after cooling it in the mold below thetemperature at which the beta modification of said orthosilicate invertsto the gamma form.

2 A mold according to claim 1, said ratio being substantially 65Ca035510:.

3. A refractory mold or mold part according to claim 1, said refractorymaterial comprising a mixture of dead burned magnesites.

4. A refractory mold or mold part for the casting of metal having atleast the metal-contacting surface portion composed of dead burnedmagnesite containing about 20 to 30 per cent of unthe highly refractorymetals in molds of investment compositions is not excluded in thepractice of this invention. It is believed by some that in the making ofdental castings, gypsum-Portland cement investments are objectionablebecause of the sulfur contained in the gypsum. An advanstabilizedcalcium orthosilicate in the beta modification and in a metastable stateand which inverts to the gamma form and undergoes dusting at the surfaceof the casting after cooling of the casting in the mold below about 675C.

RAYMOND E. BIRCH.

REFERENCES CITED The following references are of record in the file ofthis patent:

edition, pages 302 and 303.

1. A REFRACTORY MOLD OR MOLD PART FOR THE CASTING OF METAL HAVING ATLEAST THE METAL-CONTACTING SURFACE PORTION COMPOSED OF REFRACTORYMATERIAL CONTAINING, BY CHEMICAL ANALYSIS, LIME AND, SILICASUBSTANTIALLY BETWEEN THE WEIGHT RATIOS 58CAO : 42SIO2 AND 69CAO :31SIO2 AND REACTIVE WHEN HEATED BY THE CAST METAL TO AT LEAST ABOUT1300*C. TO PROVIDE AT LEAST ABOUT 10 PER CENT OF UNSTABILIZED CALCIUMORTHOSILICATE BASED ON THE WEIGHT OF SAID MATERIAL, SAID ORTHOSILICATEUNDERGOING DUSTING AT THE SURFACE OF THE CASTING AFTER COOLING IT IN THEMOLD BELOW THE TEMPERATURE AT WHICH THE BETA MODIFICATION OF SAIDORTHOSILICATE INVERTS TO THE GAMMA FORM.